This invention relates to a method and its apparatus for detecting a secondary electron emitted from a specimen, such as an LSI or photomask, irradiated by a focused charged particle beam to inspect a minute portion of it or to process it by etching or deposition.
In using focused ion beam apparatus, imbalance between income of charged energy beam and outgo of secondary charged particles on a surface of an insulating layer causes charge build-up This phenomena produces a problem of deterioration of the positioning accuracy of the charged energy beam on the surface and of electrostatic discharge.
In Japanese patent application laid open No. 3-138846 (prior art 1), a focused ion beam apparatus is described. The apparatus comprises an ion-beam irradiating means for irradiating focused ion-beam on a substrate, an electron beam source launching a low energy electron beam, a neutralizing means for neutralizing the surface charge caused by the incident ion beam by irradiating the electron beam around the irradiate portion on the surface of the substrate, and an controlling means which controls the electron beam for the neutralization on the basis of detected secondary charged particles emanated from the substrate.
And, in Japanese patent application laid open No. 60-200449 (prior art 2), it is described that a charge build-up of a specimen to be processed by irradiating an ion beam to dope into it is prevented without influencing the vicinity by irradiating an negative ion beam to neutralize in accordance with the amount of secondary electron emanated from the specimen or the amount of doped ion. And in case of irradiation of a focused ion beam, it is need to detect and observe the surface of a specimen by detecting a secondary electron emanated from it.
In Japanese patent application laid open No. 01-30155 (prior art 3), a method for preventing an electrostatic discharge of a sample by implanted ion in an ion implanting process is described. It says that in implanting ion into a sample, which is a mask a pattern of an insulation material on a silicon (Si) substrate, the sample is irradiated with another charged beam such as an electron beam, a positive ion beam. By irradiating like this, an electrical resistance of the insulating material on the silicon substrate is decreased and the implanted ion is discharged by flowing through a thickness direction of the insulating material.
A method of forming a pattern on a resist film by irradiating a charged particle beam on it is described in a Japanese patent application laid open No. 63-272034 (prior art 4). And a technic for preventing charge build-up of the resist film by irradiation of a charged particle beam is achieved by changing the electrical resistivity of it with a irradiation of low energy ion.
In general, as is described in the prior art 1, in a focused ion beam processing system, image of a sample is obtained by extracting secondary electron emitted from the surface of the specimen with a irradiation of a focused ion beam.
Also in the prior art 1, secondary charged particles produced by the irradiation of a focused ion beam are detected and the neutralizing electron beam is controlled based on the detection.
In this method to control the neutralizing electron beam, the relation between the amount of the irradiated ion beam and of the emanated secondary charged particle is determined experimentally. And, in practice, it is very difficult to control the electron beam for neutralization because the above relationship is easily influenced by the difference of the material and by the cleanliness of the surface of the specimen.
And in general, to detect a secondary ion, a front electrode of a charged particle detector is supplied a negative voltage. On the other hand, to detect a secondary electron, a front electrode of a charged particle detector is supplied a positive voltage.
In the prior art 1, when detecting the secondary ion, the electron beam for neutralization is irradiated during the detection. And when detecting the secondary electron, the electron beam for neutralization is irradiated mutually with a focused ion beam to prevent the neutralizing electron beam extracted to a charged particle detector during the detection.
Also in the prior art 2, when detecting the secondary ion, the electron beam for neutralization is irradiated during the detection. And when detecting the secondary electron, irradiation of the electron beam for neutralization is stopped during the detection.
As is discussed above, neutralizing technique by irradiating an electron beam or negative ion beam to a specimen to prevent charge build-up on the surface caused by the irradiation of a focused ion beam is described in the prior art 1 and 2. But, from our experiment, it involves a problem that it is very hard to detect a secondary electron during the irradiation of the ion beam and a very complex control is need.
In the prior art 3 and 4, a technique for preventing a charge build-up of a insulating film by reducing a resistivity of the film is described. But they are silent about detecting secondary electron to obtain a high resolution image to inspect and process the surface of the specimen by a focused charged particle beam.